In situ combustion for oil recovery in tar sands, oil shales and conventional petroleum reservoirs



Aprll 7, 1964 F, H. POETTMANN ETAL 3,127,935

1N SITU coMEUsToN FoR OIL RECOVERY 1N TAR SANDS, on. sHALEs ANDcoNvENToNAE PETROLEUM RESERVOIRS Filed April 8, 1960 fg 3 INvENToRs,FRED H. POETTMANN BERT K.LARK|N HARRY W, MILTON jl: JOHN C.BIXELATTORNEYS United States Patent O 3,127,935 IN SlTU COMBUSTION FOR OILRECOVERY IN TAR SANDS, OIL SHALES AND CONVENTIONAL PETROLEUM RESERVOIRSFred H. Poettnlann and Bert K. Larkin, Littleton, and Harry W. Milton,Jr., and John C. Bixel, Denver, Colo., assigner-s to Marathon OilCompany, Findlay, Ohio, a corporation of Ohio Filed Apr. 8, 196i), Ser.No. 21,043 19 Claims. (Cl. 1mi- 11) This invention relates tounderground combustion processes in porous media having organic content,such as oil reservoirs and fractured oil shale structures. Oil shale hasno fluid hydrocarbon present and in its natural formation is notsuficiently permeable to support in situ combustion. Consequently, thearea being worked requires some type of initial preparation orconditioning, such as shattering or fracturing. These procedures arewell known in the art and any suitable method may be utilized. Thepractice of the present invention as applied to oil shale, involvestreatment of a porous media, however produced.

This invention also is useful in the treatment of tar sands andpetroleum reservoirs, particularly those containing heavy oils. It mayutilize but is not limited to a two-pass combustion front movementthrough the structure being worked, and this may involve either separatefront movements in the same direction, or reverse combustion followed byforward combustion, as preferred.

The practice of utilizing a reverse combustion as a first pass followedby introduction of additional oxygen to reverse the front movement anddirect it in concurrent ow with the gas is well known. One of thedefects is that the increase in available oxygen produces highertemperatures with the result that the residual coke or other hydrocarbonremaining on the rocks or sand grains after the first pass is burned inthe second pass to such an extent that a substantial amount of valuablematerial is lost, which otherwise might be a product of the extraction.v

Our invention departs from prior practices in several respects, all ofwhich improve the eiciency of the treatment and provide higher yields.Of perhaps greatest importance is the reduction of available oxygenafter the initial temperature limits have been reached so as to permitcontinuing extraction without oxidation, due to the elimination ofcombustion, or through utilization of gas combustion at temperaturesbelow the minimum for burning coke, with product removal by the inertgas flow to the production well.

In either oil reservoirs or oil shale structures, a two pass treatmentmay be utilized in which the front movement of both passes is in thesame direction. To do this, surface changes are made to convert a formerinjection well or wells into a production well or wells and a formerproduction well into an injection well to direct flow into one or aplurality of production Wells which were injection wells. Particularlyin oil shale, this is of considerable value in that the rst pass frontmovement may be controlled to move rapidly through the deposit atsubstantially non-coking temperatures to bring the temperature in thearea being worked to a distilling temperature, and in so doing, thepermeability of the area is increased substantially.

3,127,935 Patented Apr. 7, 1964 Due to the fact that there issubstantially no heat loss in the oil shale deposit, the second passmovement is beneted by holding combustion to a minimum. When reversingheat front movement is employed, additional oxygen is introduced tocause the front reversal when it approaches or reaches the injectionwells and the eifect of this is to produce excessively high temperaturesand sustain combustion in the second pass. The optimum condition isattained if suflicient heat is available to distill the kerogen contentfor its removal as an oil. A substantial ga-s flow is required as theconveying vehicle, but combustion no longer is required after theminimum temperature requirement is established.

Consequently, this practice of the present invention permits effectiveutilization of the established heat and utilizes a ow of gas from a hotzone to the ultimate discharge point. The heat loss due to lowertemperatures of the injected gas is almost a negligible factor anddistillation continues over a long period after combustion isterminated. lBy eliminating secondary burning of carbonaceous matter, asubstantially greater proportion of the kerogen content is recovered asoil than is possible in a treatment where two stage combustion isutilized over a substantial time interval.

Another feature of the practice of the present invention is theutilization of accelerated frontal velocity providing what may be termeda high velocity front movement. An advantage of this velocity control isto maintain lower maximum temperatures in the area being worked and byproper control of gas velocity, gas composition and catalyst, frontaltemperatures and velocities may be maintained at predeterminedstandards. With rapid front movement, the available oxygen is rapidlyremoved from the areas where distilling temperatures persist, and at thecompletion of the initial pass, the introduction of inert or relativelyinert gases provides a control against detrimental combustion in thesecond stage treatment.

Another innovation of the present invention is the selection and use ofcatalysts permitting temperature development by gas combustion ratherthan burning a substantial amount of the organic content in attainingthe required temperature.

It is an object of our invention to provide a simple, efcient andeconomical in situ combustion treatment in porous media having organiccontent which gives a higher recovery.

Another object of our invention is to provide a simple, economical andefficient in situ combustion treatment for oil shale deposits whichutilizes a high velocity combustion front movement to initially attaindistillation ternperatures within the area being worked and thereaftermaintains distillation conditions over a protracted period Withoutrepetition of combustion.

-A furthe-r object of .the invention is to provide a simple, economicaland efficient .treatment in porous media havring organic content inwhich an initial combustion front movement is `directed through the areabeing worked in yan atmospheric having available oxygen in substantialquantity, and fol-lowing such initial front movement with a secondfron-t movement comprising gas combustion maintained at temperatures atwhich deposited coke is not burned to any substantial degree.

The accompanying drawings illustrates typical instala lations tor thepractice of our invention. In the drawlngs:

FIG. l is a schematic plan View of a typical iive-spot Iwell patternutilized in a treatment according to the present invention;

=FIG. 2 is a schematic top plan View lof another Well pattern :which maybe utilized in the practice of your invention; and

FIG. 3 is a vertical section .through a typical deposit in which thetreatment oi the present invention is practiced and illustrates in moreor less schematic arrangement, the components of the surface andunderground installations.

When a five-spot Well pattern of the type shown in FIG. l is employed, aseries of injection Wells Will be utilized 4to introduce air .or otheroxygen-containing gas into a structure being worked, represented ingeneral area by the circumferential line 7, land these injection Wells 5`direct flow of gases ltoward a substantially central production well 6.Usually in initiating combustion in such .a structure, substantialamounts `of oxygen Will be introduced through the production vvell 6 tocondition a circumferential area 8 for initiating combustion, and yafterbeing properly conditioned, combustion will be initiated through Well 6in any suitable manner, causing a heat front movement away from well 6and toward the injection Wells 5. When combustion is progressingsatisfactorily, air is introduced .through the injection Wells 5 andsupply through Well 6 is terminated with the result that reversecombustion is maintained lwith the hot gases passing the combustionfront to carry evolved products to and out of production well 6.

The arrangement shown in FIG. 2 involves linear flow paths With a row ofinjection Wells 5a supplying the air or other oxygen-containing gas anda row of production 'wells 6a arranged at substantially the same spacingas .the wells 5a with heat iront movement from the producftion Wellstoward the injection wells. After an operation `of this type iscompleted, the surface installation can be changed so that the Wells 5abecome production wells `6a and fron-t movement is continued in the sameway through an additional area to be Worked. FIG. 3 illustrates `anarrangement of :Wells in the form-ation which may be either thefive-spot pattern of FIG. 1 or the linear pattern of FIG. 2. The surfaceis indicated at 12 and a porous media to be Worked is shown at 13 -Withoverburden 14 overlying the porous media and such overburden may be ofvarying composition as indicated Iby the shading dilerences. One or moreinjection Wells 15, here shown as one, extend 'from a point above thesurface through the overburden 14 to the bottom of the porous medialayer 113. One or more production Wells, here shown as a single well`16, extend from a point above the surface to the bottom of the porousmedia structure y13:. A lateral line 17 -is provided to conduct theoutput of Well 16 past la normally open valve 18 into a collection andstorage system 19. Oxygen-containing gas, preferably a fuel and catalystmix-ture, as will be described hereinafter, is delivered through asupply -line Z1 past a normally open valve 22 into the injection well orwells and this supply is established and maintained throughout thecourse of the front movement.

In initiating combustion, it will be satisfactory to deliver .the oxygencontent through production well 16 in which event valve 1S is closed,valve 22 is closed and air or other oxygen-containing gas is deliveredfrom a source of supply 23` past an open valve 24 in line 25 fordischarge at ythe bottom of Well 16. When sufficient oxygen isintroduced in this manner to support combustion, the structure adjoiningWell 16 is ignited by any suitable method initiating a `combustion frontmovement represented at 27 which travels toward the injection Well orfwells 15. After front movement is progressing satisfactorily, the airsupply through line 25 will be terminated by closing valve Z4, valve 1.8will be open to deliver the output of Well 16 into collection system 19,and tuel and catalyst or other oxygen supply will resume through line 21by opening of valve 212.

This `arrangement serves to maintain a iioW in media `13 from theinjection wells to the production Well With a reveiise heat frontmovement maintained to provide the desired recovery. Hot gases passingyfront 27 carry products ot combustion and .other evolved materials intoand through the well 16 for delivery into the collection system 19.

When combustion iront movement is initiated adjacent production Well 16,it is preferable to arrange the associated injection well 15 as anexhaust or production Well. In such event, valve 2.2 will be closed sothat all outflow from :Well `15 Will be directed into a line 26 past anopen valve 2S for discharge into the collection System A19. With such anarrangement, it is possible to sarnple the composition of the gaspassing from Well 15, and when an appreciable amount of CO2 gas isnoted, the combustion .front movement will have progressed to a pointWhere it is advantageous `to rearrange the Wells, mak-ing Well 15 .theinjection well yand Well 16 the production fvvell. vIn such event, valve23 is closed, valve 18 is `open and valve 22, is .open to deliver ueland catalyst lfroni supply 20 into well 15 While the outflow of Well 116will pass through line 17 into collection system 19.

The feature of reversing the Well functioning may be utilizedeffectively in a later stage of the treatment. For example, if porousmedia 13 represents an oil shale deposit, such deposit will be suitablyprepared for in situ combustion by shattering of the structure accordingto Well known methods so that sucient permeability will be provided topermit the flow of gases through the area being worked and thus permitdevelopment and maintenance of a combustion front as required. Theportions of the shale structure 13x shown in FIG. 3 represent pyrolyzedor partially pyrolyzed shale and the position of the combustion front 27between the Wells 15 and 16 is indicated. The shale 13 in advance of thefront will be shattered, but at its natural temperature, Whereas thepyrolyzed shale section 13x between front 27 and Well 16 will be a hightemperature zone in Which combustion has terminated, but distillationtemperatures are maintained to continue extract of shale oil from thekerogen of the deposit.

The practice of the present invention contemplates a departure fromprior practices, in that the initial combustion front movementrepresented by 27 is maintained under temperature control to producedistillation temperatures Within a non-coking temperature range and themovement of the front through the shale effectively increasespermeability so that as the combustion progresses beyond a point wheredistillation continues, the iiow of gases into and through the hot zoneserves to maintain the required distillation temperature and to carryproducts of combustion and other evolved matter to the production Welland then to the surface. Coking temperatures for tar sands, oil shalesand conventional oil reservoirs vary widely according to diiierent oilproperties and coking rates. As noted in the May 1960, Journal ofPetroleum Technology, page 14, coking of heavy oils of tar sands willcommence at 450 to 500 F., while in relinery operations coking willbegin at S50-900 F., as noted n Advances in Petroleum Chemistry andRefining 2, page 371 (1959) Interscience Publishers, Inc., New York. Thenon-Coking limits for various materials are well known in the art andavailable in numerous publications, and therefore have not been detailedherein.

When the heat front movement has progressed to a point adjoining theinjection Well, the function of the Wells may be reversed by surfacechanges involving closing of valves 18 and 22 and opening of valves 24and 28. As a consequence, well 16 then becomes an injection well andWell 15 becomes a production well discharging through line 26 past openvalve 28 into collection system 19. At this stage, it is preferable tointroduce a fuel and catalyst mixture, the details of which will bedescribed hereinafter, and direct the flow of such gas into thepyrolyzed shale 13x from the bottom of well 16, causing it to flowtoward well 15 now functioning as the production Well. As there isalmost no heat loss to surrounding structure, the required distillationtemperatures are maintained in the material behind the advancing initialcombustion front movement and the fuel and catalyst mixture provides aconvenient means for directing a second combustion front movement fromwell 16 to well 15, which is primarily gas combustion with temperaturesmaintained sufficiently low that no appreciable amount of valuableconstituent is consumed by such front movement. Any reabsorbed shale oilfrom the first front movement is directed through the hot zone inadvance of the second front and carries to the then production well 15for delivery into collection system 19 and such second movement may be aforward or reverse movement. This procedure eliminates the excessivecombustion effect which results from reversing heat front movement andallows a sufficient time interval at any point in the area being Workedbetween the contact with the first heat front and the contact with thesecond heat front to permit a substantial amount of distillation tocontinue in such interval.

This procedure permits utilization of special controls between thetermination of the first heat front pass and the start of the secondfront pass. For example, an inert gas may be injected to reducetemperature to a predetermined value and at a predetermined rate forholding the desired heat. Inert gases from the products of combustion,nitrogen, steam or CO2 may be used. When inert gas alone is circulated,distillation continues with product removal and no combustion ismaintained in the area being worked. By introducing such gas at a highervelocity than in the first combustion front movement, an acceleratedtemperature reduction is attained with product removal continuing aspreviously described.

By utilizing a mixture of such gas and a liquid such as Water, an evenmore rapid temperature reduction may be effected. Organic liquids alsomay be used in lieu of water, which have the further advantage ofdiluting heavy immobile oils of the deposit to provide sufficientmobility to direct their removal in free-flowing condition to theproduction Well.

The preceding description refers to treatments in porous media havingorganic content, and this terminology is intended to include tar sands,oil shale deposits and conventional petroleum reservoirs containingheavy oils. While oil shale in its natural formation is not permeable,it can be rendered sufliciently permeable by fracturing or shattering topermit distribution and flow of gases through the shattered portion tosuch a degree that it becomes4 permeable porous media within therequirements of this invention. V

In its general application, the present invention provides an innovationin in situ combustion practices through controls which eliminate therequirement for burning all of the coke deposited on the sand grains.This is of particular importance in heavy oil reservoirs where the fuellaid down may be as high as three to tive pounds per cubic foot ofreservoir space. Such control is provided by use of certain air-gasmixtures, alone, or in conjunction with certain catalysts. The rate ofcombustion front movement is controlled as detailed hereinafter (column6, lines 32-4-3) and a substantial amount of gas combustion issubstituted to reduce consumption of contained fuel. The gas combustionis maintained at noncoking temperatures and with a lesser oxygenrequirement.

The heavy metal oxidation catalysts are Well suited for use in thepractice of this invention. These include copper, silver, platinum,vanadium, iron, manganese, nickel, cobalt, chromium, molybdenum, osmiumand titanium. Many of these will be present in the sandstones andassociated waters of underground porous media having organic content,but usually not in sutiicient concentration and chemical composition tobe effective in the treatment. Consequently, it is advantageous tointroduce a sufficient quantity of catalyst prior to or during thecombustion front movement to satisfy the requirements of the operation.

Various gaseous mixtures may be utilized in the practice of ourinvention, including the use of inert gases at certain stages of thetreatment. Such mixtures may include air and natural gas, air andbutane, air and propane, air and methane, air and ethane, air andacetylene, or complex mixtures of the foregoing. The gaseous mixture maybe fed separately and such feed may be continuous or intermittent. Witha distribution system such as shown in FIG. 3, it will be preferable toinitially supply a catalyst composition or mixture from storage supplystation 20 in water as a carrier vehicle through the several injectionWells 15 for distribution throughout area 7. The required property forthe catalyst seems to be that it forms unstable oxides which can beoxidized from one state to another. It also is possible to inject asolution containing a soluble catalyst salt. The salt would decompose toan oxide when heated by the approaching combustion front. Gaseouscarrier vehicles and catalysts may be used and have the advantage ofbringing the catalyst to a treatment site at the time its use isrequired. When used, such compositions will comprise the gasintroduction through an injection well, and such supply may beintermittent or continuous.

After the catalyst has been distributed as aforesaid, the porous mediais charged with sufficient oxygen to establish and maintain combustion,usually through production Well 16 as previously described. Combustionis initiated in the manner described hereinbefore and both frontaltemperature and velocity are controlled by varying total gas velocity,gas composition and catalyst. It is desirable to have a high frontalvelocity in this stage, and We have found that by proper conrol inconjunction with catalyst use, frontal velocities may be increased asmuch as two and one-half times normal velocities and with a lowering offrontal temperatures.

The use of the catalyst will be effective in oil reservoirs, tar sandsand oil shale deposits. Utilization of a rapid combustion front advancein the first pass treatment with temperature of the front maintained atessentially a noncoking temperature benefits the operation in changingthe pour point of the heavy oil causing it to flow freely to theproduction well. However, combustion moves fast enough that burning ofcontained organic content is avoided to a large degree, and yet producessufficient heat so that cracking and extraction may continue for a longiuterval after passage of the front. The continuous flew of hot gasesand products of combustion from the front to the production well offsetsany tendency toward heat loss in the area in between and makes possiblea considerable amount of cracking or distillation after combustion isterminated.

Another feature of the practice of this invention as applied to oilshale structures is the recognition that until a temperature of aboutll00 F. is attained, there is no CO2 evolution from the carbonateminerals of the deposit. Therefore, it is possible to utilize a firstfront pass to develop the main oil shale extraction with distillationtemperatures maintained after passage of the front to attain anessentially complete extraction from various parts of the working areabefore arrival of a second front. The second combustion front will becontrolled to develop temperatures approximating l500 F. so as t0 obtainCO2 extraction for process use. The second front movement may involvereversing of frontal movement as previously described, or changing wellsas previously described so as to establish uniform treatment timebetween frontal passes throughout the entire area.

lthe downstream side of the front.

It is also intended that retort off gases may be utilized in such anoperation. They are introduced at substantially higher temperatuers thanthe air normally supplied and preheat the portion of the shale inadvance of the front. While the fuel value of the return gas is usuallynot enough to be significant, establishing a high pressure preheatedzone in advance of the front assists in developing the rapid frontaladvance and assists distillation on In this way, only a negligibleamount of the extracted organic content is used as fuel and most of itpasses to the production well and is recovered in the treatment.

The catalysts described hereinbefore may be utilized in other types oftreatments with effective results. For instance, in reservoirs having noappreciable quantity of catalyst material occurring naturally, theintroduction of such catalysts will serve to reduce temperature in thecombustion front with increased velocity of its movement, all of whichproduces higher extraction. Also, in light oil reservoirs, the reducedignition temperatures deriving from catalyst introduction make otherwisevolatile constituents available as fuel and support in situ combustionwhere it would not be feasible to use it otherwise.

What we claim is:

1. In a method for the recovery of hydrocarbons from undergroundformations penetrated at spaced points by an injection well and aproducing well wherein combustion is initiated in such a formationprepared for in situ combustion and a combustion front is propagatedthrough a prepared portion of said formation having an oxidationcatalyst distributed therein, the improvement which comprisespropagating a combustion front through the formation by varying, inrelation to oxidation catalyst concentration in the prepared portion,the velocity and oxygen content of a continuous flow of a gas mixtureintroduced through the injection well for directing said frontalmovement, so as to establish a maximum temperature in said frontalmovement capable of maintaining distillation temperatures butinsufficient to cause coking of an appreciable amount of thehydrocarbons within the prepared portion during recovery of a majorportion of the hydrocarbon content of said prepared portion, andrecovering evolved products through the producing well.

2. A process as defined in claim 1, in which the ca-talyst is a coppercomposition.

3. A process as defined in claim 1, in which the catalyst is an ironcomposition.

4. A process as defined in claim l, in which the catalyst is a manganesecomposition.

5. In a method for the recovery of hydrocarbons from undergroundformations penetrated at spaced points by an injection well and aproducing well wherein combustion is initiated in such a formationprepared for in situ combustion and a combustion front is propagatedthrough a prepared portion of said formation having an oxidationcatalyst distributed therein, the improvement which comprisespropagating a combustion front through the formation, inclusive of gascombustion, by varying, in relation to oxidation catalyst concentrationin the prepared portion, the velocity and oxygen content of a continuousflow of a gas mixture introduced through the injection well forsupporting combustion and directing said frontal movement, so as toestablish a maximum temperature in said frontal movement capable ofmaintaining distillation temperatures but insufficient to cause cokingof an appreciable amount of the hydrocarbons within the prepared portionduring recovery of a major portion of the hydrocarbon content of saidprepared portion, and recovering evolved products through the producingwell.

6. In a method for the recovery of hydrocarbons from undergroundformations penetrated at spaced points by an injection well and aproducing well wherein combustion is initiated in such a formationprepared for in situ combustion and a combustion front is propagatedthrough a prepared portion of said formation having an Voxidationcatalyst distributed therein, the improvement which comprisespropagating a rst combustion front through the formation by varying, inrelation to oxidation catalyst concentration in the prepared portion,the velocity and oxygen content of a continuous flow of a gas mixtureintroduced through the injection well for directing said frontalmovement, so as to establish a maximum temperature in said frontalmovement insufficient to cause coking of any appreciable amount of thehydrocarbons within the prepared portion, directing a second combustionfront movement through said prepared portion by introduction through theinjection well of a continuous flow of a gas mixture capable ofestablishing distillation temperatures in a combustion-free atmospherein advance of said second front movement, and recovering evolvedproducts through the producing well.

7. In a method for the recovery of hydrocarbons from undergroundformations penetrated at spaced points by an injection well and aproducing well wherein combustion is initiated in such a formationprepared for in situ combustion and a combustion front is propagatedthrough a prepared portion of said formation having an oxidationcatalyst distributed therein, the improvement which comprisespropagating a combustion front through the formation by varying, inrelation to oxidation catalyst concentration in the prepared portion,the velocity, oxidation catalyst and oxygen content of a continuous owof a gas mixture introduced through the injection well for directingsaid frontal movement, so as to establish a maximum temperature in saidfrontal movement capable of maintaining distillation temperatures butinsulcient to cause coking of an appreciable amount of the hydrocarbonsWithin the prepared portion during recovery of a major portion of thehydrocarbon content of said prepared portion, and recovering evolvedproducts through the producing well.

8. In a method for the recovery of hydrocarbons from undergroundformations penetrated at spaced points by an injection well and aproducing well wherein combustion is initiated in such a formationprepared for in situ combustion and a combustion front is propagatedthrough a prepared portion of said formation having an oxidationcatalyst distributed therein, the improvement which comprisespropagating a combustion front through the formation by varying, inrelation to oxidation catalyst concentration in the prepared portion,the input velocity and oxygen content of a combustion-supporting gasmixture introduced through the injection well so as to establish amaximum temperature in the frontal movement capable of maintainingdistillation temperatures but insufficient to cause coking of anappreciable amount of the hydrocarbons within the prepared portion ofthe formation during recovery of a major portion of the hydrocarboncontent of said prepared portion, and recovering evolved productsthrough the producing well.

9. In a method for underground combustion in porous media having anorganic content, said media being penetrated at spaced points by aninjection well and a producing well, the improvement which comprisesinjecting an oxygen-containing gas through one of said wells into saidmedia, initiating a moving Zone of combustion therein at a point remotefrom the injection well, supplying oxygen-containing gas carrying anoxidation catalyst through the injection well at a regulated velocitycapable of maintaining said combustion zone at an accelerated rate ofadvance toward said injection well and at a non-coking temperature untilsaid zone has reached an area adjacent the injection well, subsequentlyintroducing an oxygendeficient fuel gas into said media for maintainingdistillation temperatures in the porous media by gas cornbustion, andremoving evolved products through said production well.

l0. In a methor of underground combustion in oil shale having an organiccontent, said shale being penetrated at spaced points by an injectionwell and a producing well, the improvement which comprises injecting anoxygen-containing gas through one of said wells into a permeable portionof the oil shale structure, initiating a moving zone of combustiontherein at a point remote from the injection Well, supplyingoxygen-containing gas carrying an oxidation catalyst through saidinjection well, varying the total oxygen-containing gas velocity andoxygencontaining gas composition so as to maintain said combustion zoneat an accelerated rate of advance toward said injection well and at anon-coking temperature until said zone has reached an area adjacent theinjection Well, subsequently introducing an oxygen-deficient fuel gasinto said shale for maintaining distillation temperatures in thepyrolyzed shale, and removing evolved products through said productionWell.

11. In a method of underground combustion in porous media having anorganic content, said media being penetrated at spaced points by aninjection well and a producing well, the improvement which comprisesinjecting an oxygen-containing gas through one of said wells into saidmedia, initiating a moving zone of combustion therein at a point remotefrom the injection Well, supplying oxygencontaining gas carrying anoxidation catalyst through the injection well at a regulated velocitycapable of maintaining said combustion zone at an accelerated rate ofadvance toward said injection well and at a noncoking temperature untilsaid zone has reached an area adjacent the injection Well, subsequentlyintroducing an inert gas into said media for maintaining distillationternperatures in the porous media by gas combustion, and removingevolved products through said production well.

12. In a method of undeground combustion in oil shale having an organiccontent, said shale being penetrated at spaced points by an injectionwell and a producing well, the improvement which comprises injecting anoxygen-containing gas through one of said Wells into a permeable portionof the oil shale structure, initiating a moving zone of combustiontherein at a point remote from the injection Well, supplyingoxygen-containing gas carrying an oxidation catalyst through saidinjection Well, varying the total oxygen-contaning gas velocity, amountof catalyst and oxygen-containing gas composition so as to maintain saidcombustion zone at an accelerated rate of advance toward said injectionwell and at a non-coking temperature until said zone has reached an areaadjacent the injection Well, subsequently introducing anoxygen-deficient fuel gas into said shale for maintaining distillationtemperatures in the pyrolyzed shale, and removing evolved productsthrough said production Well.

13. In a method of underground combustion in porous media having anorganic content, said media being penetrated at spaced points by aninjection well and a producing well, the improvement which comprisesinjecting an oxygen-containing gas through one of said wells into such aporous media, initiating a moving zone of combustion therein at a pointremote from the injection Well, supplying oxygen-containing gas carryingan oxidation catalyst through said injection Well, varying the totaloxygen-containing gas velocity, amount of catalyst and oxygen-containinggas composition so as to maintain said combustion zone at an acceleratedrate of advance toward said injection well and at a non-Cokingtemperature until said zone has reached an area adjacent the injectionwell, subsequently introducing an oxygen-deficient fuel gas into saidmedia for maintaining distillation temperatures in the media, andremoving evolved products through said production Well.

14. In a method of underground combustion in porous media having anorganic content, said media being penetrated at spaced points by aninjection well and a producing Well, the improvement which comprisesinjecting an oxygen-containing gas through one of said Wells into such aporous media, initiating a moving zone of combustion therein at a pointremote from the injection Well, supplying oxygen-containing gas carryingan oxidation catalyst through said injection well, varying the totaloxygen-containing gas velocity, amount of catalyst and oxygen-containinggas composition so as to maintain said combustion zone at an acceleratedrate of advance toward said injection Well and at a non-cokingtemperature until said zone has reached an area adjacent the injectionwell, subsequently introducing an inert gas into said media formaintaining distillation temperatures in the media, and removing evolvedproducts through said production well.

15. In a method of underground combustion in porous media having anorganic content, said media being penetrated at spaced points by aninjection Well and a producing well, the improvement Which comprisesinjecting an oxygen-containing gas through one of said Wells into such aporous media, initiating a moving zone of combustion therein at a pointremote from the injection Well, supplying oxygen-containing gas carryinga vapor phase combustion catalyst through said injection Well, varyingthe total oxygen-containing gas velocity, amount of catalyst andoxygen-containing gas composition so as to maintain said combustion zoneat an accelerated rate of advance toward said injection Well and at anon-coking temperature until said zone has reached an area adjacent theinjection Well, subsequently introducing an inert gas into said mediafor maintaining distillation temperatures in the media, and removingevolved products through said production Well.

16. In a method of underground combustion in porous media having anorganic content, said media being penetrated at spaced points by aninjection Well and a producing well, the improvement which comprisesinjecting an oxygen-containing gas through one of said Wells into saidmedia, initiating a moving zone of combustion therein at a point remotefrom the injection Well, supplying oxygencontaining gas carrying anoxidation catalyst through the injection well at a regulated velocitycapable of maintaining said combustion zone at an accelerated rate ofadvance toward said injection Well and at a non-coking ternperatureuntil said zone has reached an area adjacent the injection Well,subsequently introducing an inert gas in a liquid carrier vehicle intosaid media for maintaining distillation temperatures in the media, andremoving evolved products through said production Well.

17. In a method of underground combustion in porous media having anorganic content, said media being penetrated at spaced points by aninjection Well and a producing Well, the improvement which comprisesinjecting an oxygen-containing gas through one of said Wells into saidmedia, initiating a moving zone of combustion therein at a point remotefrom the injection Well, supplying oxygencontaining gas carrying anoxidation catalyst through the injection Well at a regulated velocitycapable of maintaining said combustion zone at an accelerated rate ofadvance toward said injection Well and at a non-coking temperature untilsaid zone has reached an area adjacent the injection Well, subsequentlyintroducing an inert gas and Water into said media for maintainingdistillation temperatures in the media, and removing evolved productsthrough said production Well.

18. In a method of underground combustion in porous media having anorganic content, said media being penetrated at spaced points by aninjection Well and a producing Well, the improvement which comprisesinjecting an oxygen-containing gas through one of said Wells into saidmedia, initiating a moving zone of combustion therein at a point remotefrom the injection Well, supplying oxygencontaining gas carrying anoxidation catalyst through the injection well at a regulated velocitycapable of'maintaining said combustion zone at an accelerated rate ofadvance toward said injection Well and at a non-coking temperature untilsaid zone has reached an area adjacent the injection Well, subsequentlyintroducing an inert gas and an organic liquid into said media formaintaining distillation temperatures in the media, and removing evolvedproducts through said production well.

19. In a method for the recovery of hydrocarbons from undergroundformations penetrated at spaced points by an injection Well and aproducing Well wherein combustion is initiated in such a formationprepared for in situ combustion and a combustion front is propagatedthrough a prepared portion of said formation having an oxidationcatalyst distributed therein, the improvemnet which cornprisespropagating a combustion front through the formation by varying, inrelation to oxidation catalyst concentration in the prepared portion,the velocity and oxygen content of a continuous flow of a gas mixtureintroduced through the injection well for directing said frontalmovement, so as to establish a maximum temperature in said frontalmovement capable of maintaining distillation temperatures but insucientto cause appreciable CO2 evolution, directing a second frontal movementthrough the prepared portion with enough oxygen-containing gas suppliedso as to obtain substantial CO2 extraction through- Vout the preparedportion, and recovering evolved products through the producing Well.

References Cited in the iile of this patent UNITED STATES PATENTS t2,788,071 Pelzer Apr. 9, 1957 2,793,696 Morse May 28, 1957 2,871,942Garrison et a1. Feb. 3, 1959 2,889,881 Trantham et al. June 9, 19592,917,112 Trantham et al. Dec. 15, 1959 2,917,296 Prentiss Dec. 15, 19592,994,374 Crawford et a1. Aug. 1, 1961 3,007,520 Frey Nov. 7, 19613,019,837 Marx et al. Feb. 6, 1962 3,048,225 Reichle Aug. 7, 1962

1. IN A METHOD FOR THE RECOVERY OF HYDROCARBONS FROM UNDERGROUNDFORMATIONS PENETRATED AT SPACED POINTS BY AN INJECTION WELL AND APRODUCING WELL WELL WHEREIN COMBUSTION IS INITIATED IN SUCH A FORMATIONPREPARED FOR IN SITU COMBUSTION AND A COMBUSTION FRONT IS PROPAGATEDTHROUGH A PREPARED PORTION OF SAID FORMATION HAVING AN OXIDATIONCATALYST DISTRIBUTED THEREIN, THE IMPROVEMENT WHICH COMPRISESPROPAGATING A COMBUSTION FRONT THROUGH THE FORMATION BY VARYING, INRELATION TO OXIDATION CATALYST CONCENTRATION IN THE PREPARED PORTION,THE VELOCITY AND OXYGEN CONTENT OF A CONTINUOUS FLOW OF A GAS MIXTUREINTRODUCED THROUGH THE INJECTION WELL FOR DIRECTING SAID FRONTALMOVEMENT, SO AS TO ESTABLISH A MAXIMUM TEMPERATURE IN SAID FRONTALMOVEMENT CAPABLE OF MAINTAINING DISTILLATION TEMPPERATURES BUTINSUFFICIENT TO CAUSE COKING OF AN APPRECIABLE AMOUNT OF THEHYDROCARBONS WITHIN THE PREPARED PORTION DURING RECOVERY OF A MAJORPORTION OF THE HYDROCARBON CONTENT OF SAID PREPARED PORTION, ANDRECOVERING EVOLVED PRODUCTS THROUGH THE PRODUCING WELL.